Vinylidene fluoride (VDF) polymers are known in the art to be suitable as binders for the manufacture of electrodes for use in non-aqueous-type electrochemical devices such as batteries, preferably secondary batteries, and electric double layer capacitors.
Generally, techniques for manufacturing electrodes involve the use of organic solvents such as N-methyl-2-pyrrolidone (NMP) for dissolving VDF polymer binders and homogenizing them with a powdery electrode material and all other suitable components to produce a paste to be applied to a metal collector.
The role of the organic solvent is typically to dissolve the VDF polymer in order to bind the powdery electrode material particles to each together and to the metal collector upon evaporation of the organic solvent.
Nevertheless, more recently, approaches are pursued wherein use of organic solvents is commonly avoided so as to ensure more environmentally friendly techniques, and waterborne solutions have been proposed.
For instance, US 2006/0099505 (SONY CORPORATION) 11 May 2006 discloses a process for manufacturing an anode for a battery comprising using an anode mixture slurry prepared by dispersing a particulate anode active material and a particulate binder containing at least one of the group consisting of homopolymers and copolymers of vinylidene fluoride (VDF) in a dispersion medium having a swelling degree of 10% or less to the binder. The dispersion medium is preferably water.
Similarly, US 2010/0304270 (ARKEMA INC.) 2 Dec. 2010 notably discloses water-borne electrode-forming compositions comprising, inter alia, vinylidene fluoride polymer particles and powdery electrode materials, including notably:                carbonaceous materials, nano-titanate or other matrix capable of being doped with lithium ions, in the case of negative electrode; and        oxides, sulfides or hydroxides of lithium and/or a transition metal (including Co, Mn, Al, Ti, Ni, Fe . . . ), in the case of positive electrode.        
Nevertheless, cathode materials are generally sensitive to moisture/humidity e.g. because of solubility in water of the same, and/or possibly hydrolysis phenomena, up to complete decomposition of the material, negatively affecting final electrode performances. Thus, the water-borne route for manufacturing electrodes may lead to poorer final battery performances or can even be an absolutely non practicable solution, when solubility and/or decomposition is at issue, because of the unavoidable presence of moisture introduced during manufacture of the electrode itself, which might aggress the active material at the high temperatures of typical operations.
There is thus still a current shortfall in the art for aqueous electrode-forming composition providing excellent performances in secondary batteries, with no negative impact due to the humidity/moisture on the active electrode material.
On the other side, techniques involving the coating or plating of positive (cathode) electrode materials for improving their performances in Li batteries, including embodiments wherein the same are used for formulating cathodes including PVDF binder, are known. Rationale for including said additional coating layer is often modifying electrical conductivity, minimize oxygen loss at high temperature (and hence minimizing explosion failures of battery), limiting dissolution of the material by interaction with the electrolytic solutions, etc. Examples of such disclosures are briefly summarized herein below.
Document JP 8138670 (TOSHIBA CORP) 31 May 1996 discloses particles of LiNiO2 having on their surface a layer comprising an additional element whose concentration is higher then in the core, said element being selected from a alkaline metal other than Li, a alkaline-earth metal, a transition metal other than Ni, a group III element, a group IV element, a group V element or a chalcogen. This layer is taught as increasing the stability of the crystal structure of the LiNiO2 core, so that cyclability of the corresponding Li battery is enhanced. Core-shell particles as above described are obtained from thermal sintering of hydroxides or other precursors mixtures: because of the formation of the crystalline latice of LiNiO2, the additional component is preferably expulsed to the surface, under the form of oxide. Exemplified embodiments make use of B, Al, Si, P, Mn, Co, Fe, Zn, Ga. As binding agent is mention made, inter alia, of PVDF, PTFE, EPDM.
Document JP 11016566 (HITACHI LTD) 22 Jan. 1999 discloses a cathode material (or a cathode assembly) covered with a metal or a metal compound having excellent oxygen uptake capability. The problem solved is controlling the ignition and explosion of a cell, by surrounding the cathode active material, which might generate oxygen under overcharging or overheating conditions, with a metal (or metal oxide compound) able to react and capture said oxygen. The metal is selected from Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, Mo. Electrodes are manufactured using PVDF as binding agent.
Document US 2002076486 (SAMSUNG SDI CO., LTD) 20 Jun. 2002 discloses a method for coating a positive electrode material including mixing and drying simultaneously a composition of cathode material and coating precursor in a liquid medium. Said liquid medium can be water or an organic solvent e.g. an alcohol. The coating element is any of Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, preferably Al or B. The coated materials were used for fabricating electrodes from a paste comprising PVDF in NMP.
Document CN 101083318 (BYD CO LTD) 5 Dec. 2007 discloses the use of certain plating compositions comprising a particular reducing agent for coating certain compound onto the surface of positive electrode materials, in particular LiFePO4 and LiCoO2. The metal used for coating can be any of Al, Mg, Ti, Cu, Ag, Ba, Ca, Mn, Fe, Co, Ni. The coated materials were notably used for fabricating electrodes from a paste comprising PVDF in NMP.
SONG, GUI-MING, et al. Enhanced electrochemical properties of LiFePO4 cathode for Li-ion batteries with amorphous NiP coating. Journal of Power Sources. 2010, vol. 195, no. 12, p. 3913-3917. discloses coating of LiFePO4 with a Ni/P alloy from an aqueous plating bath. Electrodes therefrom were manufactured using PVDF as a binder.